Using electrical probes, the scientists recorded directly from the brains of female voles as they encountered a potential partner, mated for the first time and began to show signs of having formed a lifelong bond, indicated by “huddling” behaviour.

Robert Liu, co-lead author also at Emory University, said: “It’s kind of like human cuddling: they like to sit side by side and veg out.”

The electrodes recorded peaks in activity in two brain areas known to be linked to the formation of a pair bond – the medial prefrontal cortex, which is involved in executive control, and the nucleus accumbens, the central hub of the brain’s reward system.

The team found that the baseline level of communication between the two regions predicted how likely a female was to form a bond, which Liu describes as “an individual predisposition to being affectionate”.

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Optogenetics is a poor choice of name for this technique but it's very promising for testing predictions.

I'll note that IBM Watson typed this crowd as very high in Openness but very low in "artistic interest", which probably stands in for Aesthetic Engagement (and possibly Affective Engagement, slightly) as defined in this article.